A display device such as a liquid crystal display device comprises liquid crystal cells which selectively transmit light, and a backlight unit which irradiates planar light on a rear surface of the liquid crystal cells.
As the backlight unit, a surface light source device comprising a light-emitting device which emits point light, first and second aspheric lenses which collimate light from the light-emitting apparatus as parallel light, a cylindrical Fresnel lens which converges the parallel light transmitted through the first and second aspheric lenses to a light guide plate, and the light guide plate which emits planar light has been proposed (for example, JP 2009-231018 A).
Generally, light emitted from a light-emitting device such as a laser diode is point light which is diffused elliptically, and has a specific aspect ratio. Meanwhile, a light guide plate is formed thin and flat in order to make a surface light source device thin.
Accordingly, there is a great difference between the aspect ratio of an incident surface of the light guide plate and the aspect ratio of a cross section of an optical path. When the light is irradiated onto the incident surface such that a horizontal beam width of the light matches a horizontal width of the incident surface, a vertical width of the optical path exceeds a vertical width of the incident surface. Since light leaking from the incident surface is not guided to an emission surface and the light is thereby lost, power is wasted. Conversely, when the light is irradiated onto the incident surface such that a vertical beam width of the light matches a vertical width of the incident surface, a horizontal width of the optical path becomes less than a horizontal width of the incident surface. Because of this, the light does not reach every corner of the light guide plate, and plane emission cannot be performed evenly.
In order to approximate the aspect ratios of the incident surface and the light, in JP 2009-231018 A, a cylindrical Fresnel lens which converges parallel light greater than the incident surface in the vertical width is added. However, even if the vertical beam width of light extending greatly is to be adjusted by a lens later as described in JP 2009-231018 A, a loss of light occurs inside the lens. Also, in JP 2009-231018 A, light is reflected multiple times within the light guide plate to guide the light. Accordingly, when polarized light is used as a light source, there is a concern of reduction of a degree of polarization.